1. Field of the Invention
The present invention relates generally to a head up display, and more particularly, to a color holographic head up display mounted on an airplane or automobile for projecting a color image displayed on a screen so that the navigator can see it along with the surrounding scenery.
2. Description of the Related Art
A holographic head up display is a kind of image projection apparatus which comprises a display plate such as a CRT or LCD (liquid crystal display) for displaying an image to be projected, a relay optical system for delivering the displayed image, and a holographic combiner which functions as an exit pupil and for redirecting the image from the relay optical system to the field of view of the navigator, thereby allowing the navigator to simultaneously see the image and the external scenery.
The head up display of an airplane utilizing the holographic combiner is well known in the art. The holographic combiner is a kind of holographic optical element which combines two different images to enable them to be seen simultaneously. The holographic combiner transmits a portion of a light as well as serving as one or more of a reflector, a spherical reflector, diffusion plate, and lens.
There are two kinds of head up displays utilizing the holographic combiner. One of them makes a projected image formed at the infinity to enable a pilot or driver to view remote objects continuously without turning his eyes elsewhere. The other head up display forms a projected image at a short distance from the pilot or driver so that the pilot or driver can obtain information necessary for navigating or driving without turning his head to a different direction from that of navigation or driving. A typical example of the head up display wherein a projected image is formed at the infinity is disclosed in U.S. Pat. No. 4,763,990.
The head up display disclosed in U.S. Pat. No. 4,763,990 will be described hereinbelow with reference to FIG. 1. FIG. 1 illustrates an optical arrangement of a holographic head up display for making a projected image formed at the infinity by utilizing a holographic combiner which serves as a spherical reflector. As illustrated in FIG. 1, the holographic head up display 100 comprises an information source 1, a relay optical system 2 for relaying the light from the information source 1, a reflector 3, and a holographic spherical reflector 5 which is a kind of holographic combiner. A CRT having a narrow spectral bandwidth may be used as the information source 1. The holographic spherical reflector 5 is made of a reflective plate transmitting the light partially or a spherical reflecting mirror. The CRT image from the information source 1 is relayed to the reflector 3 via the relay optical system 2 and then converged to the focal plane of the holographic spherical reflector 5 as a first image 4. This first image 4 is relayed to the holographic spherical reflector 5 so that a second image is formed at the infinity. The image from an exit pupil of the holographic spherical reflector 5 forms the so called viewing zone or eye box where the navigator can view remote objects through a projected image. The viewer 7 can see the second image of the CRT image and the surrounding scenery simultaneously through the eye box.
However, only the light with a small angular divergence can be collimated after reflection by the holographic spherical reflector 5 and incident upon the eye box because the holographic spherical reflector 5 has a very high angular selectivity below several degrees. Since the width of the collimated beam is lower than 10 cm and is consistent with the dimension of the image 6 of the exit pupil in a typical head up display, the viewing zone, i.e., the eye box is very narrow. In addition, only monochromatic images can be displayable since the holographic spherical reflector 5 has a high spectral selectivity, i.e., the reflection bandwidth is ordinarily within 15-30 nm. Recently, this type of head up display is rarely used because of the difficulty in manufacturing the reflection type holographic screen for viewing color images.
Next, the head up display disclosed in U.S. Pat. No. 5,037,166 wherein a projected image is formed at a short distance from the viewer will be described with reference to FIG. 2. FIG. 2 illustrates an optical arrangement of a holographic head up display for making a projected image formed around the viewer by utilizing a holographic diffusion screen 9 as a holographic combiner. The arrangement of this holographic head up display 200 is similar to that shown in FIG. 1 and thus the detailed description thereof will be omitted. The display image 8 from the information source 1 is relayed to the reflector 3 via the relay optical system 2. The relayed image is then reflected by the reflector 3 and projected to the holographic diffusion screen 9. The projected image is placed in the eye box 10 after being scattered at each point on the holographic diffusion screen 9. In this case, the size of the eye box 10 is larger than that in FIG. 1 utilizing the holographic spherical reflector as a holographic combiner. The projection of color images can be achieved when the holographic diffusion screen is used since the reflected spectral bandwidth of the holographic diffusion screen is broader than that of the holographic spherical reflector.
Although the size of the eye box of the head up display is extended by using the holographic diffusion screen, the diffraction efficiency becomes smaller with the result that the brightness of an image is reduced. In addition, scattering is increased because of the noise effect produced by the intermodulation between object beams since the diffusion screen is a hologram of the object having a diffusion surface such as a ground glass. As a result, the tone of an image is deteriorated since a halo occurs in the image displayed on the diffusion screen when the image is not projected onto the holographic diffusion screen. In general, when the holographic diffusion screen is utilized as a holographic combiner, the display of a color image is not viable due to a spectrum dispersion. Nontheless, it is theoretically possible to display color images if the spectrum dispersion can be made great. However, in this case, the diffraction efficiency is reduced.